The present disclosure relates to a light guide member for guiding an illumination light emitted from a light source, an illuminating device using the foregoing light guide member, and an image reading apparatus and an image forming apparatus using the same.
An image forming apparatus such as a scanner or a copy machine uses an illuminating device, which irradiates light on a document sheet, in order to optically read an image of the document sheet. In recent years, a white light emitting diode (LED) is being used as the light source of the illuminating device due to its advantage of having high luminous efficiency. With this type of illuminating device, it is necessary to linearly illuminate the document sheet. Therefore, a bar-shaped light guide member and the white LED are combined to generate a linear illumination light since an LED is a point light source. The light guide member includes an entrance surface which is disposed at one end of the light guide member and through which the illumination light emitted by the white LED enters, a strip-shaped exit surface which extends in the longitudinal direction of the light guide member and outputs the illumination light therefrom, and a strip-shaped reflecting surface which is formed on a face of the light guide member, the face being placed opposite to the exit surface and which reflects the illumination light.
The illumination light that entered the entrance surface propagates within the light guide member and is output to the outside from the exit surface. This outgoing light includes an illumination light (direct light) that heads directly from the white LED to the reflecting surface and then is reflected off the reflecting surface, and an illumination light (indirect light) which heads toward the reflecting surface after being totally reflected one or more times by a peripheral surface of the light guide member and that is reflected off the reflecting surface. The direct light is mainly output from a portion near the entrance surface. Here, since the direct light and the indirect light have different illumination intensity, there is a problem in that the uniformity of the illumination light cannot be obtained in the longitudinal direction of the light guide member. In order to resolve this problem, with a conventional illuminating device, the cross section shape of the light guide member is formed in a polygonal shape, and the shape of its main scanning direction is caused to be different so that the direct light is not output from the exit surface.
Nevertheless, in order to generate an indirect light, the illumination light emitted from the white LED needs to be totally reflected at least one by the peripheral surface of the light guide member. Consequently, with the light guide member of a conventional illuminating device, a certain length of distance needs to be provided between the near ends of the entrance surface and the exit surface. This leads to the entire length of the light guide member being extended, and inhibits the miniaturization of the illuminating device.